1. Field of the Invention
The present invention relates to catalysts for molding compounds. More particularly, the present invention relates to latent catalysts for epoxy molding compositions, which are particularly useful as encapsulants for electronic packaging materials.
2. Brief Description of Related Technology
Epoxy resin compositions are widely used for electronic packaging materials in the electronics industry, and, in particular, as encapsulants for semiconductor elements and electronic circuits. Compositions used as electronic packaging materials must have high reliability including excellent thermal cycle resistance due to extensive temperature changes encountered through typical use of electronic devices. For example, assembly of electronic components typically involves exposure of the electronic component to high temperatures to achieve solder reflow for establishing electrical interconnection between a chip and a substrate, as well as to achieve proper curing of any polymeric material which may be used as an underfill material between the chip and the substrate or as an adhesive for adhering the chip to the substrate.
Accordingly, epoxy resin based compositions, and, in particular, cresol novolac-type epoxy compositions, have been widely used in the formation of molding compositions for use as electronic packaging materials. Such epoxy resin based compositions typically include an epoxy resin, a curing agent such as a phenolic hardener, and a bulk amount of a filler component, as well as other additives. In addition, curing catalysts are typically included within the composition for promoting reaction of the epoxy resin.
Recently, the trend in the industry is to develop molding compounds that can withstand higher solder reflow temperatures. For example, diphenyl oligomers are currently preferred over dianhydride oligomers and diamine oligomers due to higher reliability. Also, fast curing catalysts, such as tertiary amine compounds, tertiary phosphine compounds and their derivatives, are typically employed in epoxy molding compounds for improving productivity reliability and hot hardness strength properties. When molding compounds incorporate resins with higher reactivity, such as the more reactive diphenyl oligomers, the amount of curing catalyst included in the molding compound must be optimized to provide optimum curing of the resin. However, when the curing catalysts are provided at such optimum levels for reliability, the curable resins within the molding compound are more reactive at lower temperatures, such as during storage. Long term shelf life stability is therefore compromised due to an increase of viscosity of the composition from the higher reactivity at temperatures below the optimum curing temperature.
U.S. Pat. No. 6,084,037 to Shimizu et al. discloses an epoxy resin composition for use as an encapsulant for semiconductor devices. The composition includes an epoxy resin, a curing agent, and a filler, with a specific curing accelerator comprising a reaction product of trimellitic anhydride and 1,8-diazabicyclo(5.4.0)undecene-7 (xe2x80x9cDBUxe2x80x9d), and a mercapto-containing silane coupling agent. Introducing such components to form the curing accelerator, however, may impart undesirable characteristics to the finally cured composition.
U.S. Pat. No. 6,458,472 Bi to Konarski et al. discloses fluxing underfill compositions useful for fluxing metal surfaces in preparation for providing an electrical connection and sealing the space between semiconductor devices. The fluxing underfill compositions include an epoxy resin, an acidic fluxing agent and an anhydride, along with a latent curing agent complex of a salt of a nitrogen-containing component and a portion of the acidic fluxing agent.
Accordingly, it would be advantageous to provide a molding composition useful as an encapsulant for electronic packaging materials that provides the advantages of improved reactivity and reliability with long term shelf life stability, and which incorporates traditional compounds within the molding composition to achieve these properties.
It has been discovered through the present invention that epoxy compositions having prolonged shelf life can be achieved through the use of a latent curative. As such, the present invention is directed to a curative which is particularly useful for accelerating the cure of epoxy compositions. The curative includes a combination of an inorganic-based carrier having an activated surface, and a catalyst compound including a moiety capable of accelerating the cure of an epoxy composition, such as a nitrogen-containing compound or a phenyl-containing compound. The inorganic carrier is subjected to conditions to activate at least a portion of its surface, such as through a heat treatment to expose active groups, such as through the removal of labile water molecules. The activated surface of the inorganic-based carrier includes a high surface area porous surface, and may include reactive surface groups which are capable of bonding to the catalyst compound through a hydrogen bond. As such, a curative may be formed as a reaction product of the catalyst compound bonded with the inorganic carrier.
The present invention is further directed to a composition including the latent curative. In particular, the present invention is further directed to a composition which includes an epoxy resin, a curing agent for the epoxy resin, and a latent curative composed of a combination of an inorganic-based carrier having an activated surface and a catalyst compound including a moiety capable of accelerating curing of the epoxy resin. Desirably, the activated surface of the inorganic-based carrier includes reactive surface groups capable of bonding to the moiety through a hydrogen bond, and further includes a high surface area porous surface, such that the catalyst compound is sorbed on the activated surface.
Desirably, the curative includes an inorganic-based carrier of silica reacted with either a phenyl-containing catalyst compound such as triphenyl phosphine or, more desirably, a nitrogen-containing catalyst compound such DBU. The epoxy resin and the curing agent are desirably highly reactive components, such as bisphenol-based epoxies and cresol-based hardeners.
The compositions or the present invention when cured are particularly useful as electronic packaging materials, and may therefore also include a bulk amount of a filler as well as other components conventionally used in such compositions.
In a further embodiment, the present invention is directed to a process for preparing a curative for an epoxy resin. The process involves activating the surface of an inorganic-based carrier, such as by removing water from the surface, for example through heating the surface under vacuum. The inorganic-based carrier is then combined with a catalyst compound having reactive moieties such that at least a portion of the moieties in the catalyst compound are deactivated. The material is then dried, thereby forming the latent curative. The reactive moieties of the catalyst compound may be deactivated through bonding with the inorganic-based carrier through a hydrogen bond to form a complex. Desirably, the catalyst compound is a nitrogen-containing catalyst compound, with at least a portion of the nitrogen atoms of the nitrogen-containing catalyst being bonded to the inorganic-based carrier through a hydrogen bond, thus forming an acid-base complex.